Low carbon high yield strength alloy steel

ABSTRACT

A LOW CARBON, LOW ALLOY STEEL COMPOSITION WHICH AFTER BEING HEAT TREATED IS CHARACTERIZED BY HIGH YIELD STRENGTH AND DUCTILITY SUITABLE FOR COLD FORMING OPERATIONS CONSISTING ESSENTIALLY BY WEIGHT OF:   CARBON: ABOUT 0.05-0.25% MANGANESE: ABOUT 0.50-1.30% SILICON: ABOUT 0.40-0.90% ZIRCONIUM: ABOUT 0.03-0.15% BORON: ABOUT 0.0005-0.0025%   WITH THE BALANCE IRON AND RESIDUAL IMPURITIES. THE HEAT TREATED STEEL COMPOSITIONS CAN BE COLD FORMED TO PRODUCE FOR EXAMPLE TRUCK FRAME RAILS.

United States Patent O 3,676,108 LOW CARBON HIGH YIELD STRENGTH ALLOY STEEL William F. McGarrity, Dearborn, Mich., assignor to National Steel Corporation No Drawing. Filed June 1, 1970, Ser. No. 42,511 Int. Cl. C22c 39/54 US. Cl. 75-123 B 7 Claims ABSTRACT OF THE DISCLOSURE A low carbon, low alloy steel composition which after being heat treated is characterized by high yield strength and ductility suitable for cold forming operations consisting essentially by weight of:

Carbon: about 0.050.25% Manganese: about 0.501.30% Silicon: about GAO-0.90% Zirconium: about 0.03-0.15 Boron: about 00005-00025 with the balance iron and residual impurities. The heat treated steel compositions can be cold formed to produce for example truck frame rails.

BACKGROUND OF THE INVENTION This invention relates to low carbon, low alloy steels. It particularly relates to a low carbon alloy steel composition wherein manganese, silicon and zirconium are utilized as alloying agents.

Such steels are well known. For example, Selmi et al. US. Pat. No. 2,234,130 discloses a low alloy high tensile strength steel consisting essentially by weight of from 0.05-0.25% carbon, from -0.500.75% manganese, from 0.70-0.90% silicon, from 0.10-0.15% zirconium, from 0.24-0.75% chromium, and from 0.02-0.50% molybdenum; desirable ductility is maintained after heat treatment as a result of the molybdenum ingredient. The Making, Shaping and Treating of Steel, 8th edition, discloses at p. 1093 quenched and tempered constructional low alloy steels sold under the designation N-A-XTRA 100 comprising by weight 0.15% carbon, 0.80% manganese, 0.70% silicon, 0.07% zirconium, 0.60% chromium, and 0.15% molybdenum, and having a minimum yield strength of 100,000 pounds per square inch. Selmi et al. US. Pat. No. 2,250,505 discloses a low alloy steel characterized by variable hardenability and fine grain structure which contains by weight from 0.25-0.70% carbon, from 0.50-0.75% manganese, from 0.70-0.90% silicon, from 0.050.35% zirconium, from 0.50-0.75% chromium, and from ODS-0.35% molybdenum. Lautle et al. US. Pat. No. 2,797,991 discloses a low alloy steel capable of being produced in the hot rolled, normalized, or cold reduced-annealed condition with a yield strength of 38,000 pounds per square inch upward and a fine grain structure which can be subjected to cold forming operations; the steel composition comprises by weight from 0.06-0.15 carbon, from 0.350.75% manganese, from GAO-0.90% silicon, from 0.05-0.15% zirconium, with a silicon to manganese weight ratio ranging from 1:1 to 1.521. Zanetti Ser. No. 659,885, filed Aug. 11, 1967 discloses a low alloy steel characterized by a tensile strength minimum of 75,000 pounds per square inch without heat treating and having a ductility suitable for structural uses which comprises by weight from 0.07-0.20% carbon, from 0.751.15% manganese, from 0.50-1.0% silicon, from 0.50-0.80% chromium, from 0.05-O.20% zirconium, from 0.0005-0.0025% boron, from 0.02-0.15 titanium, and from 0.010.6% columbium.

It is an object of the present invention to provide a low carbon alloy steel composition which can be heat treated so as to be characterized by a yield strength minimum of 75,000 pounds per square inch and ductility such that it can be cold formed wherein an almost infinitesimal amount of boron is utilized as the key alloying agent and wherein expensive alloying agents such as chromium and/ or columbium need not be utilized to provide the high yield strength nor molybdenum to achieve the desired ductility. It is a further object of the present invention to provide a low carbon alloy steel composition which can be heat treated so that it can be cold-formed to produce products such as truck frame rails or which can be normalized or otherwise heat treated to provide pipeline stock.

Crafts US. Pat. No. 2,280,283 broadly discloses a steel composition comprising by weight from 0.1-l% carbon, up to 2% manganese, up to 2% silicon, from 0.03l% zirconium, and from 0.0005-0.05% boron. In all the specific examples in Crafts, boron is included at the 0.01% by weight level. With a level of 0.01% by weight boron, the steels disclosed in Crafts when heat treated to provide a yield strength in excess of 75,000 pounds per square inch are not sufiiciently ductile so that they can be cold formed. In particular the compositions of the Crafts specific examples when heat treated to provide a yield strength in excess of 75,000 pounds per square inch cannot be cold formed to provide products such as truck frame rails. Thus Crafts does not disclose any specific composition which satisfies the requirements of the aforestated objects.

Moreover, Grainal alloys which contain boron have been used as intensifiers for controlling and increasing the capacity of steels to harden. However, Grainal alloys have not heretofore been known to have been used to produce the steel composition of this invention.

DETAILED DESCRIPTION OF THE INVENTION Carbon: about 0.05-0.25 Manganese: about 0.50 -1.30% Silicon: about 0.40-0.90% Zirconium: about 003-0. 15% Boron: about 0.0005-0.0025% the balance being iron and residual impurities such as are ordinarily encountered in conventional basic oxygen plant, open hearth or electric furnace practice used in producing the steel.

The maximum desirable values of the most common residual impurities encountered are:-

Weight percent maximum Phosphorus 0.035 Sulfur 0.04 Copper 0.35 Aluminum 0.07

This steel composition is capable of being heat treated to possess a minimum yield strength of 75,000 pounds per square inch while retaining ductility such that it can be readily cold formed to provide products such as truck frame rails. The composition can be normalized or otherwise heat treated to provide pipeline stock. The composition additionally has the advantage of being weldable.

The upper limit for carbon is ordinarily not exceeded in order to maintain the good weldability of the end product. A carbon content below the lower limit stated above is ordinarily avoided in order to have suificient carbon available for providing desired tensile strength. Preferably, the composition comprises from about 0.15 to about 0.20% by weight carbon.

The manganese is included to increase the yield point, tensile strength and cold formability. Preferably, the composition comprises by weight about 0.75% to about 1.10% manganese. The manganese can be added in the furnace or in the ladle. Where possible for example, in open hearth practice it is added in both the furnace and the ladle.

The silicon component acts as a deoxidizer and ferrite strengthener. Preferably, the composition comprises by weight from about 0.60 to about 0.85% silicon. It can be added in the furnace or in the ladle; where practicable silicon is added in both the furnace and in the ladle.

The zirconium component acts to refine the grain structure, increase hardenability, increase toughness, and increase cold formability. Preferably the composition comprises by weight from about 0.075% to about 0.125% zirconium. The zirconium is added in the ladle.

It is absolutely essential for the provision of the aforedescribed benefits of the present invention that the amount of boron ingredient be kept within the aforedescribed limits. The use of boron within these limits acts to substantially increase yield strength of the heat treated prodnot without substantially detracting from the ducility of that product. If less than about 0.0005 weight percent boron is utilized, the yield strength of the heat treated product is ordinarily less than 75,000 pounds per square inch. If the amount of boron exceeds the aforestated upper limit of about 0.0025 weight percent, the ductility of the heat treated product can be unacceptable. Preferably the composition of this invention comprises from about 0.0010 to about 0.0015 weight percent boron.

The boron can be added in any convenient form. For example, it can be introduced into the steel together with silicon deoxidizing agent in the form, for instance, of Grainal 79 It can also be added separately into the molten steel. It can be added for example in the form of borax, calcium borate, boron carbide, or as ferroboron. It is usually added either in the ladle or in the mold. In a continuous casting process preferably it is added in the ladle.

The aforedescribed composition of the present invention is heat treated to provide a product having a minimum yield strength of 75,000 pounds per square inch while retaining ductility so that it can be cold formed. The heat treating process can comprise for example, heating the aforedescribed composition for example in bar, plate, sheet or other forms to a temperature above the austenizing temperature; then quenching to ambient temperature; then tempering at a temperature ranging from about 800 F. to about l300 F.the higher the tempering temperature is within this range, the higher the ductility is and the lower the yield strength is. Preferably, in the heat treating process the aforedescribed composition is maintained at a temperature ranging from about 1600 F. to about 1800 F. for a time period ranging from about 15 minutes to about 45 minutes; water quenched; and then tempered at a temperature ranging from about 850 F. to about 1 Grainal 79 is an alloy available commercially from Vanaldiam pofi-poration of America having the following composition y weig With the balance iron.

Grainal 79 addition is an excellent way to add the boron since the aluminum in the Grainal 79 protects th boron from oxidation.

105 0 F. for a time period ranging from about 15 minutes to about 45 minutes.

The heat treated product even in the form of a plate of one inch thickness is readily cold formed, for example either by roll forming or by press working. The heat treated product can be cold formed for example a press brake to provide truck frame rails. The heat treated cold formed product has a minimum yield strength of 75,000 pounds per square inch. It has the additional advantage of a minimum tensile strength of 85,000 pounds per square inch and is readily weldable.

The following example further illustrates a steel composition within the scope of the present invention, the heat treatment of plates of this composition to provide a yield strength in excess of 75,000 pounds per square inch and the cold forming of said plates to provide structural products such as frame rails suitable for use for truck frames.

EXAMPLE Low carbon, boron treated, alloy steel plates are prepared having the following composition by weight.

Component: Weight percentage Carbon 0.19 Manganese 1.05 Phosphorus 0.01 1 Sulfur 0.016 Silicon 0.79 Zirconium 0.09 Boron 0.0012 Iron and residual impurities balance REPRESENTATIVE TEST DATA Yield Elongationstrength initial gage 0.2% Tensile length of ofiset strength 2 inches Brinell Direction (p.s.i.) (p.s.i.) (percent) hardness Longitudinal 128, 830 137, 830 20 285 Transverse 126, 200 135, 530 18 293 The satisfactory ductility of the heat treated product is indicated by the elongation results. The heat treated product can be satisfactorily roll-formed or press-worked to a three times thickness radius. The heat treated product can be readily cold formed on a press brake to produce high strength structural products such as truck frame rails.

Equal results of yield strength and cold formability are obtained when the boron is added as ferroboron in the mold instead of as Grainal 79 in the ladle.

The invention can be embodied in other specific forms without departing from the spirit or the essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

1. A loW alloy steel composition which when heat treated to possess a minimum yield strength of 75,000 pounds per square inch retains ductility suitable for cold forming operations consisting essentially by weight of Carbon: about 0.05-0.25% Manganese: about 0.50-1.30% Silicon: about 0.400.90% Zirconium: about 0.030.15% Boron: about 0.000S0.0025% with the balance iron and residual impurities.

2. A low carbon alloy steel composition as recited in claim 1 consisting essentially by weight of:

Carbon: about 0.150.20% Manganese: about 0.75-1.10% Silicon: about 0.70-0.85% Zirconium: about 0.0750.125% Boron: about 0.0010-0.00*15% with the balance iron and residual impurities.

3. A low carbon alloy steel composition as recited in claim 1 which has been heat treated to possess a minimum yield strength of 75,000 pounds per square inch.

4. An article which has been cold formed from the heat treated product of claim 3.

5. A truck frame rail which has been cold formed from the heat treated product of claim 3.

6. A method for producing a low carbon alloy steel composition characterized by a minimum yield strength of 75,000 pounds per square inch and ductility suitable for cold forming operations comprising the steps of (a) providing an alloy steel composition consisting essentially by weight of:

6 Carbon: about 0.05-0.25 Manganese: about 0.50l.30% Silicon: about 0.40.90% Zirconium: about 0.03-0.15 Boron: about 0.0005-0.0025% with the balance iron and residual impurities;

(b) heating said composition to a temperature above the austenizing temperature;

(0) then quenching;

(d) then tempering at a temperature ranging from about 800 F. to about 1300 F.

7. A process as recited in claim 6 wherein in step (b) the steel composition is maintained at a temperature rang ing from about 1600 F. to about 1800 F. for a time period ranging from about 15 minutes to about minutes; wherein in step (c) the quenching is water quenching; and wherein in step (d) the tempering is carried out at a temperature ranging from about 850 F. to about 1050 F. for a time period ranging from about 15 minutes to about 45 minutes.

References Cited UNITED STATES PATENTS 2,280,283 4/1942 Crafts 123 B 2,797,991 7/1957 Laufie 75-123 H 2,871,117 1/1959 Korchynsky 75123 B L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner U.S. Cl. X.R. 

